This invention relates to klystron vacuum tubes and in particular to so called external cavity klystron vacuum tubes.
Part of a typical external cavity klystron vacuum tube arrangement as at present known is illustrated in FIG. 1.
Referring to FIG. 1 the tube comprises a vacuum wall section 1 in the form of a dielectric cylinder transparent to electro-magnetic radiation. This forms a window through which power is effectively transmitted. The cylindrical section 1 has, at either end, cylindrical copper annuli 2,3 which are shaped to receive the ends of the cylindrical wall member 1 and provide, by portions of increased diameter, contact for spring fingers 4 attached to the inner rims of a box member 5 surrounding the cylindrical wall 1 and forming the external cavity. In order to adjust the resonant frequency of the external cavity, a pair of moveable tuning plungers (extending at right angles to the plane of the paper and not shown) is provided, one on each side of the cylindrical wall member 1. To the left (as shown) of the cylindrical wall member 1 is a coupling loop 7, adjustable by rotation, by which output power is coupled to an external transmission line and aerial (not shown).
Particular attention should be given to the method of forming the vacuum seal between the cylindrical wall member 1 and the copper annuli 2,3. Since the cylindrical wall member 1 is normally of a high purity alumina or beryllia ceramic consistent with its function as a window, there will be observed a differential expansion between the cylindrical member 1 and the copper annuli 2,3. The coefficient of expansion of the material of the cylindrical wall member 1 is very much less than that of copper.
In order to accommodate for this differential expansion the joint between the cylindrical wall member 1 and each of the copper annuli 2,3 is normally as shown in more detail in FIG. 2.
Referring to FIG. 2 it will be seen that the cylindrical wall member 1 is located within a recess 8 in the copper annulus 3 (with a similar arrangement at the other end of the member 1). Use is made of an inner cylindrical flare 9 and an outer cylindrical flare 10. Inner flare 9 is brazed at one end 11 between the end of the ceramic wall member 1 and a ceramic balance ring member 12. The outer flare member 10 is brazed to the outermost wall of the recess 8 in the copper annulus 3. Finally the vacuum seal is completed by welding inner flare member 9 to outer flare member 10.
The whole construction forms a vacuum joint with the balance ring 12 pressing against the copper disc 3 and taking up the axial thrust due to the external pressure when the tube is under vacuum. In addition the balance ring 12 forms a sliding abutment with the base of the recess 8 accommodating for differential expansion between the cylindrical wall member 1 and the copper disc 3.
It has been found that a klystron constructed as described with reference to FIGS. 1 and 2 suffers from a serious defect. The output cavity 5 operates at the highest power level and the peak radio frequency voltage across the cavity is approximately the same as the operating voltage of the klystron, typically between 20 and 26 kV for a high power television klystron. It has been found that arcing sometimes occurs in the cavity which not only can cause puncturing of the seal provided by the flares 9 and 10 but also, of course, interruption of operation if, as is commonly the case, an arc detector is introduced which removes the r.f drive from the klystron upon the detection of an arc.
The present invention seeks to provide an improved construction in which the aforementioned problem is mitigated.